I`m not sure if youre right..
I use three GeForce 8800GTX Cards (each 768MB) and an Ageia PhysiX Accelerator (3x PCIe for GeForce, 1x PCI for Ageia).
The GeForce itself has one SLi connector (for the second card). If you bridge card one and two, theres no bridge free for the third one! The Ageia card is for what you discribed: Extra calculation (physical and mass environment) but do not need to be connected physical with the GeForce (done internal via bus).
The third GeForce will be always a standalone (independent) card (imho). I don`t know a way, to use triple Geforce 8.. even none for GeForce 7 .. (think it can be done only by external hardware), or is there a way to enable SLI functions without a hardware bridge between them?

But nevermind, this is not the main problem :P (more important: to get the boxing chars synchronized).